Heat and pressure consolidated decorative laminates have been produced commercially for a number of years, and have found widespread acceptance as mar-resistant surfaces for wallcoverings, paneling, tabletops, countertops, and the like. These laminates contain a number of laminae that are consolidated to form a unitary structure carrying a surface decoration which can range from something as simple as a solid color to something as complex as an embossed simulated wood grain finish.
Although the methods of preparing such laminates and the number and types of laminae can vary widely, the procedure for preparing such laminates generally involves the consolidation of one or more sheets of core stock, depending primarily on the ultimate thickness desired, in combination with a decorative or print sheet and a top coat or overlay. The core stock usually comprises an unbleached kraft paper which has been impregnated with a relatively inexpensive thermosetting resin, such as a phenolic resin, which is easily cross-linked upon the application of pressure and heat. In order to prevent sticking of the laminate to the press platens specially treated caulstock or a release medium such as a paper or foil with a release coating surface are used between the platens and the laminate.
The decorative or print sheet has more stringent requirements than the core stock. It is usually pigment filled, must be capable of being impregnated with a noble thermosetting resin cross-linkable upon the application of heat and pressure, but exhibiting no color deterioration upon the application of such heat and pressure, and it must not allow any strike-through or bleeding of the resin used in the core stock. In many instances, the decorative or print sheet must also be capable of being printed with a design, such as simulated wood grain, which survives the consolidation step intact. Two of the most common noble thermosetting resins used to impregnate the decorative or print sheet are urea-formaldehyde resin condensates and melamine-formaldehyde resin condensates. Other resins such as polyester resins have also been so employed.
In most instances an overlay sheet is superimposed over the decorative or print sheet prior to consolidation of the laminae. The overlay sheet is generally a thin, high-quality, alpha-cellulose paper which is also impregnated with a noble thermosetting resin, usually the same type in the decorative or print sheet. The overlay sheet is usually transparentized during the consolidation step, thereby enabling the decoration and/or printing which is present on the decorative or print sheet to be readily seen in the finished laminate, yet imparting a greater degree of mar and abrasion resistance to the decorative laminate than would otherwise be obtained without such an overlay.
An alternative to using an overlay sheet is to employ a polymeric film between the decor sheet and the release medium. U.S. Pat. No. 3,616,021 discloses the use of a thermoplastic film in this manner. Films used in this manner are likely to be thermoplastic materials such as acrylics. Since the film is self-supporting it is necessarily thick and because of its rigid structure, the film is very brittle and handling of the film is difficult. Contamination of the film is also a problem since dust is picked up very easily by both sides of the film.
Another alternative to using an overlay sheet is to employ a film of noble thermosetting resin on a substrate having release properties. This approach provides compatibility between the resins in the film and the resin in the decorative sheet, but not a reaction between the resins of the film and the decorative sheet to produce a laminate with the improved surface properties of the invention. DISCLOSURE OF THE INVENTION
The present invention is a process for preparing a decorative laminate comprising the steps of:
(A) stacking together in an assembly PA0 (B) applying heat and pressure to the assembly to result in PA0 (C) separating the releasing carrier surface from the unitary structure.
(a) one or more resin impregnated core sheets, PA1 (b) a decorative fibrous sheet impregnated with a noble thermosetting resin, and PA1 (c) a releasing carrier surface coated with a film composition comprising polyvinyl butyral resin and melamine resin in contact with the decorative fibrous sheet; PA1 (a) the film composition on the releasing carrier surface and the noble thermosetting resin of the decorative fibrous sheet reacting with each other and PA1 (b) the laminae being consolidated into a unitary structure; and
The invention is also the decorative laminate produced by the above described method.
The invention is also a transferable film composition on a releasing carrier surface capable of transferring to and reacting with noble thermosetting resin impregnants of decorative fibrous sheets in a heat and pressure laminating process, the film composition comprising of polyvinyl butyral resin and melamine resin.
The present invention provides decorative laminates with improved bond between the decorative fibrous sheets and the superimposed film surfacing compositions because of the reactivity between the two. It also can provide other desirable surface properties, such as weather resistance or decorative effects, without adversely affecting the bond properties.
The essential components of the film composition on the releasing carrier surface are thermosetting polyvinyl butyral resin and melamine resin. Polyvinyl butyral has the ability to thermoset into a hard durable film and also reacts with melamine resin. Polyvinyl butyral is a polyvinyl acetal prepared by reacting butyraldehyde with polyvinyl alcohol.
Although polyvinyl acetal resins normally are thermoplastic and soluble in a range of solvents, they may be cross-linked through heating with a trace of mineral acid as a catalyst. Cross-linking is thought to be caused by trans-acetalization, but may also involve more complex mechanisms such as a reaction between acetate or hydroxyl groups on adjacent chains. As a practical matter, cross-linking of the polyvinyl acetals is carried out by reaction with various thermosetting resins such as phenolics, epoxies, ureas, diisocyanates and melamines. The availability of the functional hydroxyl groups in vinyl acetals for condensations of this kind is an important consideration of this application. Incorporation of even a small amount of vinyl acetal resin into thermosetting compositions markedly improves toughness, flexibility and adhesion of the cured coating.
Vinyl acetal films by themselves are characterized by high resistance to aliphatic hydrocarbons, mineral, animal and vegetable oils (with the exception of castor and blown oils). They withstand strong alkalis but are subject to some attack by strong acids. However, when employed as components of cured coatings, their stability to acids as well as solvents and other chemicals is improved greatly.
The presence of hydroxyl groups in the acetal polymer molecule not only enables good wetting of most substrates, particularly important when applying the composition to a release coated temporary carrier, but also furnishes reactive sites for chemical combination with other thermosetting resins.
Thus, in the presence of an acidic catalyst such as p-toluene-sulfonic acid, condensation reactions take place on heating among all of the reactive functional groups of the polyvinyl acetal of the transfer film, the melamine resin of the transfer film and the melamine resin of the decorative sheet (groups such as hydroxyl, amine, methoxy and acetal) to produce a very strong, cross-linked matrix. The reaction product is particularly stable when the polyvinyl acetal resin of the present invention is employed, for this resin has a long carbon chain backbone that would not be present if only melamine-formaldehyde resins were used.
By including a melamine resin in the film composition, the benefits are further enhanced since the kinetics of the reaction are greatly improved due to the intimate mixing of the components. In addition to participating in the foregoing reactions, melamine resin improves the solvent resistance of the transfer film. It removes the film from the thermoplastic state, and the entire film becomes one that is basically thermoset. The mechanical properties of the film are also greatly improved. It should be noted that when melamine alone is used as a transfer film it is quite brittle and unmanageable.